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Tue, 13 Jan 2026 09:30:49 +0000 (GMT) Message-ID: <4bd0e615-193d-47c6-8933-a93d75a2f29c@linux.ibm.com> Date: Tue, 13 Jan 2026 15:00:49 +0530 Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH v4 3/3] sched/fair: Remove nohz.nr_cpus and use weight of cpumask instead To: Vincent Guittot Cc: mingo@kernel.org, peterz@infradead.org, linux-kernel@vger.kernel.org, kprateek.nayak@amd.com, juri.lelli@redhat.com, vschneid@redhat.com, tglx@kernel.org, dietmar.eggemann@arm.com, anna-maria@linutronix.de, frederic@kernel.org, wangyang.guo@intel.com References: <20260112050442.138446-1-sshegde@linux.ibm.com> <20260112050442.138446-4-sshegde@linux.ibm.com> From: Shrikanth Hegde Content-Language: en-US In-Reply-To: Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-TM-AS-GCONF: 00 X-Proofpoint-GUID: f-CJBxScdoJw7VJI0Qgc7DW-YUurNSuK X-Proofpoint-ORIG-GUID: f-CJBxScdoJw7VJI0Qgc7DW-YUurNSuK X-Authority-Analysis: v=2.4 cv=TaibdBQh c=1 sm=1 tr=0 ts=696610cf cx=c_pps a=bLidbwmWQ0KltjZqbj+ezA==:117 a=bLidbwmWQ0KltjZqbj+ezA==:17 a=IkcTkHD0fZMA:10 a=vUbySO9Y5rIA:10 a=VkNPw1HP01LnGYTKEx00:22 a=VnNF1IyMAAAA:8 a=7m7kHNSTB5oX0r7sibsA:9 a=QEXdDO2ut3YA:10 X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwMTEzMDA3NyBTYWx0ZWRfX+gkB1lseNxXk SYN4XPJduA67ON33M35gtv6Ik6htqtCi//2bD3U4/x0QgZj0PTNiUIItqBdlDZVFvyMKoP9fZd3 ZL+ai7i+QBvZ66uzU9wVkFTGnuBuqZveF1NEVgaOGzV6ETc1uWYSlCvslz4r6ltAAgLIOtNZEM0 a1Gy7jn9nzctnao3X8MuWie7zjgueDoquxk6Dj4v13f/flPAU1mrOqhoeIJTyfh+ucrTo/BD5jK R/rCt08BPvZrjMo0ejy3aG4QmWOhk3z0D/Mfj96u5WVCBryvJ+w+n61meFgi0RVGbyZY9HiDWXP QISB59H+fU2JejXXfWPQz2V0rSciub60LGtxCclc/VMFPH1EyhBStiHj6tYtOO/O3Omf3S1n1V2 uirn1b/v5efJ6IOWhBTmhzFWvBjc0mx+ZLnCGtAfEfKk1Bdp2OMQruk5SnDUzjXlY8WwT/rmRpz XXRMVojMUEYD+buksFQ== X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1121,Hydra:6.1.9,FMLib:17.12.100.49 definitions=2026-01-13_01,2026-01-09_02,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 adultscore=0 phishscore=0 impostorscore=0 bulkscore=0 clxscore=1015 suspectscore=0 priorityscore=1501 malwarescore=0 lowpriorityscore=0 spamscore=0 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.19.0-2512120000 definitions=main-2601130077 Hi Vincent. >> On system with 480 CPUs, running "hackbench 40 process 10000 loops" >> (Avg of 3 runs) >> baseline: >> 0.81% hackbench [k] nohz_balance_exit_idle >> 0.21% hackbench [k] nohz_balancer_kick >> 0.09% swapper [k] nohz_run_idle_balance >> >> With patch: >> 0.35% hackbench [k] nohz_balance_exit_idle >> 0.09% hackbench [k] nohz_balancer_kick >> 0.07% swapper [k] nohz_run_idle_balance >> >> [Ingo Molnar: scalability analysis changlog] >> Signed-off-by: Shrikanth Hegde > > This change makes sense to me but I'm not convinced by patch1. > You wrote in patch 1 that It doesn't provide any real benefit, what > are the figures with only patch 3 ? > Whole point of patch 1 is, (assuming normal case i.e system wont be 100% busy) - we read the value and then do time check. - we bail out if time is not due. Why bother reading, if time is not due. I won't expect patch1 to make any major difference. But seemed like right thing to do, considering that most of the time system won't be 100% busy. So numbers will be same without patch1. >> --- >> kernel/sched/fair.c | 5 +---- >> 1 file changed, 1 insertion(+), 4 deletions(-) >> >> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c >> index c03f963f6216..3408a5beb95b 100644 >> --- a/kernel/sched/fair.c >> +++ b/kernel/sched/fair.c >> @@ -7144,7 +7144,6 @@ static DEFINE_PER_CPU(cpumask_var_t, should_we_balance_tmpmask); >> >> static struct { >> cpumask_var_t idle_cpus_mask; >> - atomic_t nr_cpus; >> int has_blocked_load; /* Idle CPUS has blocked load */ >> int needs_update; /* Newly idle CPUs need their next_balance collated */ >> unsigned long next_balance; /* in jiffy units */ >> @@ -12466,7 +12465,7 @@ static void nohz_balancer_kick(struct rq *rq) >> * None are in tickless mode and hence no need for NOHZ idle load >> * balancing >> */ >> - if (unlikely(!atomic_read(&nohz.nr_cpus))) >> + if (unlikely(cpumask_empty(nohz.idle_cpus_mask))) >> return; >> >> if (rq->nr_running >= 2) { >> @@ -12579,7 +12578,6 @@ void nohz_balance_exit_idle(struct rq *rq) >> >> rq->nohz_tick_stopped = 0; >> cpumask_clear_cpu(rq->cpu, nohz.idle_cpus_mask); >> - atomic_dec(&nohz.nr_cpus); >> >> set_cpu_sd_state_busy(rq->cpu); >> } >> @@ -12637,7 +12635,6 @@ void nohz_balance_enter_idle(int cpu) >> rq->nohz_tick_stopped = 1; >> >> cpumask_set_cpu(cpu, nohz.idle_cpus_mask); >> - atomic_inc(&nohz.nr_cpus); >> >> /* >> * Ensures that if nohz_idle_balance() fails to observe our >> -- >> 2.47.3 >>